Synthesis and characterization of indolocarbazole-quinoxalines with flat rigid structure for sensing fluoride and acetate anions

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Abstract
A new series of indolocarbazole-quinoxalines (ICQ, receptors 6 and 7) are prepared and characterized for effective fluoride and acetate anion sensing. The new indole-based system has a highly flat rigid structure with a large π system, and exhibits high binding affinity and sensitivity for acetate and fluoride anions. Receptors 6 and 7 give abundant and unique spectral features in dimethyl sulfoxide (DMSO). Both fluoride and acetate anions cause a bathochromic shift of the absorption peaks of receptor 7 in DMSO, whereas only fluoride anion results in a remarkable shift of the absorption peak of receptor 6 in DMSO. Receptors 6 and 7 can also operate as efficient colorimetric sensors for naked-eye detection of fluoride and acetate anions, and their combined use also offers a simple way for distinguishing these two anions by the naked-eye. The analysis of a Job's plot for the binding of receptor 7 and F−, single crystal structures of 7·TBACl and 7·TBACH3COO confirm 1 : 1 binding stoichiometry. Notably, the ICQ system offers novel and excellent receptors for acetate anion both in solution and in crystalline solid through the formation of two hydrogen bonds.